Small, efficient, durable planar fluorescent lamps are useful in many applications, including head-mounted displays such as those used in virtual reality headsets. Important considerations in such applications are the efficiency of the lamp, the uniformity of illumination provided by the lamp and lifetime of the lamp.
Inefficient lamps require high power input to attain an adequate level of illumination. However, operation at high power levels is undesirable for several reasons. For example, the cost and complexity of voltage sources is increased as the voltage level increases. In many applications, a high voltage input may also pose a shock hazard to a user. Further, at higher voltages, sputtering of electrodes may be increased reducing the lifetime of the lamp.
Nonuniformity of illumination is also undesirable. Lack of uniformity degrades the appearance of the lamp. Moreover, uneven illumination can cause backlit displays to appear uneven and, in some cases, render portions of the display unreadable.
Inadequate lifetimes of such lamps increases the cost of providing a light source. Also, where such lamps are used as part of a larger unit, failure of the lamps may require disassembly or replacement of a much larger unit. For example, where such lamps are used as backlights for flat panel displays, their replacement can be difficult and expensive.
As is known, efficiency of fluorescent lamps can be improved by increasing the discharge length between electrodes. However, in small lamps, the achievable discharge length is restricted by the small dimensions of the lamps. To lengthen the effective discharge length and thereby improve the efficiency of such lamps, serpentine discharge paths are sometimes defined within the lamps to cause the electrical discharge to follow a serpentine pathway between the electrodes.
Even where a serpentine path is used, efficiencies of the lamps are often quite low. To further improve their efficiency, such lamps can also be operated at temperatures well above ambient temperature. However, running the lamps at high temperatures is disadvantageous for several reasons. First, the heat generated by the lamp can make the lamps uncomfortable or dangerous in head-mounted displays such as virtual reality headsets. Also, operating lamps at such high temperatures consumes excess energy, thereby reducing the efficiency of the lamps. Further, operating lamps at higher temperatures accelerates aging of the lamps, causing premature failure.
The lifetime of small planar lamps can be affected by several factors. As discussed above, operation at higher temperatures reduces the lifetime of the lamp, principally by inducing unnecessary sputtering of the electrodes. Also, heating and cooling of the lamp can cause mechanical failure of the lamp due to a variety of factors. For example, heat-induced expansion and contraction can stress joints in the lamp or cause flexure of portions of the lamp, as is known. High voltage and high temperature may also induce migration of ions through materials forming the lamp. These ions may, in turn, form conductive paths, or shortcuts, across the lamp, which can impair the uniformity and efficiency of the lamp.